The present invention relates to a rubber composition, comprising two kinds of styrene-butadiene copolymer rubber different from each other in their styrene content and the vinyl bond content in their butadiene units polybutadiene rubber, natural rubber and/or synthetic polyisoprene rubber, and more particularly, to a rubber composition for tire tread having excellent abrasion resistance, wet skid resistance, and rolling resistance.
Rubber compositions capable of reducing rolling resistance have been proposed in the art from the viewpoint of fuel saving (e.g., U.S. Pat. Nos. 4,334,567, 4,485,205). Since these rubber compositions are each mainly composed of so-called "high-vinyl styrene-butadiene copolymer rubber" having a relatively high vinyl bond content (hereinafter referred to as "HV-SBR"), the use thereof for tire tread had the disadvantage of poor abrasion resistance although it offered a relatively good balance between rolling resistance and wet skid resistance with respect to gripping performance on a wet road surface.
In order to eliminate the disadvantage of poor abrasion resistance, e.g., Japanese Patent Laid-Open Nos. 70134/1982, 87441/1982, and 21435/1983 each propose a method wherein use is made of a combination of HV-SBR with natural rubber and polybutadiene rubber having a low vinyl bond content. However, these methods had a disadvantage that the wet skid resistance was inevitably lowered to some extent. Further, insufficient wet skid resistance could be attained by the method disclosed in Japanese Patent Laid-Open No. 87443/1982 wherein use is made of a combination of HV-SBR with styrene-butadiene copolymer rubber having a low vinyl bond content, natural rubber and/or polybutadiene rubber having a low vinyl bond content.
A rubber composition disclosed in Japanese Patent Laid-Open No. 207342/1987 comprising styrene-butadiene copolymer having a high styrene content and a low vinyl bond content can offset the above-described disadvantage to some extent. However, the effect is not yet satisfactory.